In 2014, we got our first ever planeswalker commanders
Thats why its been a classic sideboard threat in formats like Modern and Legacy for years, and its why getting Blood Mooned is basically shorthand for being locked out of casting your spells
Do you want to be on the attack and linear, or do you want to be reactive and more flexible
You see, not only does she halve any damage dealt to your or your permanents, but she also doubles the damage you deal to your opponents and their permanents
Finally, the ultimate isn't even good all the timeif you are winning and have a decent board, you might want to keep your own permanents rather than take your opponent's, especially since Aminatou, the Fateshifter is more about synergy than raw power, so your cards will likely be more valuable to your deck than your opponent's cards
There are five to collect, with decklists available here